Shale oil production



United States Patent 3,1 13,620 12/1963 Hemminger lnventor Barry W. Parker Bartlesville, Okla. Appl. No. 691,959 Filed Dec. 20, 1967 Patented Dec. 29, 1970 Assignee Phillips Petroleum Company a corporation of Delaware SHALE 01L PRODUCTION 10 Claims, No Drawings 0.5. CI. 166/303, 166/247, [66/259, 166/272 Int. Cl EZlb 43 24, E2 1 b 43/26 Field Search 166/1 1, 36, 39, 40, 42, 272, 247, 303, 256, 259, 308

References Cited UNITED STATES PATENTS 3,284,281 1 1/ 1966 Thomas 166/40X 3,303,881 2/1967 Dixon 1 166/36 3,342,257 9/1967 Jacobs et a1. 166/11 3,404,919 10/1968 Dixon ll 166/36X OTHER REFERENCES Lombard, Recovering Oil From Shale With Nuclear Explosives" Journal of Petroleum Technology, Vol. XVll, No. 8, August 1965, (pp. 877-882). Copy in 166-36 U.N.E.

Primary Examiner-Stephen J. Novosad Attorney-Young and Quigg 1 ISHALE OIL PRODUCTION BACKGROUND OF THE INVENTION The use of nuclear explosivesto fragment underground formations has gained considerable acceptance as an economically feasible method of producing oil and gas from reservoirs having such low original permeability as to be incapable of economic production in the original state. The utilization of nuclear explosives in this regard is described briefly by D. B. Lombard in his article Recovering Oil from Shale with Nuclear Explosives" published in Aug. 1965 issue of Journal of Petroleum Technology, pages 877-882.

By this method a nuclear charge is placed at the desired elevation in a suitable reservoir strata and detonated to produce a cavity containing fragmented reservoir rock; the dimensions of the cavity and the extent of fragmentation depending, of course, upon the magnitude of the detonation and the characteristics of the surrounding formations. For example, Lombard refers to the effects of detonating nuclear charges having energies of from to about 100 kilotons in hard rock and indicates that the resulting fragmentation zone; i.e., nuclear chimney, has a diameter of from 100 to several hundred feet and a vertical extent of about 2% cavity diameters measured from the point of detonation to the chimney top.

It is further pointed out by Lombard that although the fragmentation zone or nuclear chimney is fairly well defined, that the sidewalls; i.e., the remaining unfragmented formation defining the nuclear chimney, possess numerous fractures extending outwardly in all directions from the sidewalls for a distance of approximately one-half of the diameter of the fragmentation zone. These fractures result in a substantial increase in the permeability of the formation surrounding the fragmentation zone which enable the egress of formation fluids from the fragmentation zone and the strata immediately surrounding the nuclear chimney during subsequent retorting operations and corollary procedures involving the use of elevated temperatures and pressures'within the fragmented area. It is, of course, desirable, to retain formation fluids within the fragmentation zone so that the ultimate recovery of these fluids is not diminished.

It is therefore one object of this invention to provide a method for treating fragmentation zones produced by subterranean detonation. It is another object of this invention to improve the production of reservoir fluids from fragmentation zones resulting from subterranean detonations. It is yet another object of this invention to provide a method for treating subterranean nuclear chimneys. It is another object of this invention to minimize the egress of reservoir fluids from subterranean-detonation zones. It is another object of this inven- 7 tion to provide a method for decreasing the permeability of formations defining the periphery of nuclear chimneys. It is yet another object of this invention to improve the ultimate recovery of reservoir fluid from strata fragmented by subterranean detonation. It is another object of this invention to provide a method for preparing steam for reducing the permeability of detonation chimney sidewalls. It is yet another object of this invention to provide a method for washing particulate and soluble material from subterranean detonation fragmentation zones. It is another object of this invention to provide a method for utilizing the heat retained in retorted subterranean fragmentation zones.

SUMMARY OF THE INVENTION In accordance with one embodiment of this invention the permeability of strata, particularly oil-shale formations, defining the periphery of fragmentation zones created by subterranean detonations is reduced by contacting the formations with steam at a temperature below the retorting temperature of fragmented shale within said fragmentation zone, yet sufficiently in excess of temperature of said formations to promote the expansion thereof, for alperiod of time sufficient to cause the desired degree of swellihg of the formations defining the fragmentation zone periphery whereby the permeability of those formations is reduced.

In accordance with another embodiment of this invention a fragmentation zone produced by a subterranean detonation is contacted with steam at a temperature sufficiently in excess of the temperature of the formations defining the periphery of the fragmentation zone to effect the expansion thereof with a consequent reduction in permeability, after which oil bearing rock fragments contained within said fragmentation zone are retorted at a temperature in excess of that employed to reduce the permeability of the surrounding strata.

In accordance with yet another embodiment of this invention the permeability of formations defining the periphery of a subterranean fragmentation zone is reduced by contacting the same with steam at a temperature sufficiently in excess of the formation temperature to effect the swelling thereof, at least the latter portion of said contacting being conducted at a pressure sufliciently in excess of formation pressure to force said steam selectively into zones of relatively higher permeability thereby effecting the selective permeability reduction of these zones.

In accordance with yet another embodiment water is injected into a previously retorted subterranean fragmentation zone containing substantial amounts of heat retained from the retorting thereof to produce steam, at elevated temperatures, which can then be injected into a fragmentation zone prior to retorting to reduce the permeability thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The degree of fracturing and consequently the degree of permeability which results in those strata defining the outer periphery of the fragmentation zone depends primarily on certain characteristics of the formations, per se, and to some extent on the intensity of the detonation. The average permeability of the periphery diminishes with distance from the fragmentation zone for any given cross-sectional area. However, a further consideration which must be taken into account in this regard is the ever expanding cross-sectional area available to flow at increasing radial dimensions outwardly from the periphery of the fragmentation zone. The fractures extending outwardly from the chimney sidewall communicate with other fractures in the formation. As a result, considerable loss of reservoir fluid will result during retorting subsequent to detonation if steps are not taken to deter the flow of reservoir fluid through the fragmentation zone periphery. Such losses of reservoir fluid can be as high as 100 percent of the otherwise recoverable material contained within the fragmentation zone. The permeability of reservoir rock immediately adjacent the fragmentation zone can be reduced by as much as percent with a consequent reduction in the loss of reservoir fluid from the fragmentation zone.

These losses are generally amplified by the methods necessarily or at least most economically employed in the production of reservoir fluid from such detonation or nuclear chimneys. For example, it has been found effective to retort the fragmented shale in such chimneys by causing the passage of a flame front from the uppermost portion of the fragmented oil shale downwardly through the entire mass of fragmented rock, whereby liberated hydrocarbons flow downwardly through the fragmentation zone and accumulate in the lower extremities thereof. The temperature of the combustion zone is controlled by recycling, for example, 3 to 5 volumes of gas for each volume of air injected. These accumulated hydrocarbons are then preferably produced by means of a production well directionally drilled downwardly and laterally into the lower extremities of the fragmented area. Such production of the accumulated fluids can be accomplished by any conventional means. These methods can be made more efficient by operating at a positive pressure over the accumulated reservoir fluid in order to prevent the vaporization thereof by the elevated temperature encountered during shale retorting. Such positive pressures also promote the egress of accumulated fluid by way of the production well referred to above. It is equally evident, however, that elevated pressures promote the migration of liberated reservoir fluid through the walls of the fragmented zone if some provision were not made to diminish the permeability thereof.

Elevated operating pressures, 100 to 1,000 p.s.i.a. are desirable in some processes to retort nuclear chimneys such as when hot shale gas is recycled to retort the nuclear chimney to reduce the cost of compressors and wells. One embodiment of the hot shale gas recycle process is described in copending application Ser. No. 641,815. The process of this invention has several advantages when employed in combination with the retorting process described in copending application Ser. No. 639,490, the shale would be preheated and the amount of air necessary to retort the chimney would be reduced.

Therefore, in accordance with this invention the fragmentation zone reaches a temperature within the range of from about 150 to about 250 F. following detonation. The period of time required to accomplish this result will vary considerably depending upon the heat transfer characteristic of the reservoir rock and the size of the charge employed. However, elapsed times of 14 to about 60 days following detonation are sufficient to allow the accomplishment of this result. Following this period of time the original borehole by which the charge is initially implanted in the proximity of the desired oil shale strata is reopened by whatever means are required, such as additional drilling due to disruption of the borehole by the effects of the detonation. Steam is then injected into the fragmentation zone at a temperature within the range of from about 350 to about 600 F and at a pressure of from about 135 to about 1,500 p.s.i.g. The rate at which the permeability of the surrounding strata is reduced will depend on the rate at which the temperature of the reservoir rock is increased. The rate of temperature increase will, of course, depend upon the size of the fragmentation zone and the rate at which heating steam is injected. It is presently preferred where the reservoir rock is oil shale that the final reservoir temperature prior to retorting approach a temperature within the range of from about 350 to about 600 F. By virtue of the differential between the this final temperature and the initial reservoir temperature after detonation and prior to steam treatment and the relatively high thermal coefficient ofexpansion for oil shales which generally approximate 30 X F this procedure will provide a 50 to 500 fold reduction in sidewall permeability.

Following this steam pretreatment to reduce the permeability of the fragmentation zone perimeter the oil shale fragments contained within the detonation chimney are retorted by any one of numerous processes with which the art is familiar. For example, suitable processes for in situ retorting of oil shale are discussed generally by D. B. Lombard in his article "Recovering Oil from Shale with Nuclear Explosives" above referred to and U.S. Pats. Nos. 3,342,257, Jacobs et al. and 3,303,881 of Dixon. These processes generally involve the ignition of the upper surface of the fragmented oil bearing rock; e.g., shale, and the passage of air downwardly through the formation to promote the gradual combustion of a portion of the hydrocarbon material contained in the reservoir rock with the consequent liberation of reservoir fluid. Recycle gas is preferably mixed with injected air to sustain a degree of combustion which will maintain the reservoir at a temperature sufficient to liberate entrapped hydrocarbons. Suitable reservoir temperatures which can be employed during such operations are generally within the range of about 650 to about 1,000 F. As pointed out previously in this specification it is also often desirable to maintain a substantial positive pressure on the reservoir during retorting to prevent excessive vaporization of reservoir fluids, and to minimize costs of wells and compressors. Suitable pressures are generally within the range of 100 to about 1,000 p.s.i.g. As a result ofthese factors and the additional consideration of the desired rate of shale retorting preferably at least about 15,000 and usually about 30,000 standard cubic feet of shale gas are injected per ton of shale retorted. Shale gas compositions suitable for most applications comprise about 1 volume of air and about 3 to about 5 volumes of recycle gas.

Another procedure found desirable for retorting the shale in the fragmentation zone is described in copending application Ser. No. 641,815. According to that process a hydrocarbon having high heat capacity is heated, generally at the surface, to a predetermined temperature and is' injected into the top of the subterranean fragmentation zone: to educe oil therefrom, The liquid and vaporous hydrocarbons flow downwardly through the fractured shale and are recovered through a well bore near the bottom of the fragmentation zone. In that process the injected hydrocarbon gases are free of oxygen with the result that there is no combustion within the shale mass, all heat being supplied by surface heating of the hydrocarbon.

The hydrocarbon gas of the process described in Ser. No. 641,815 can be substituted with steam or mixtures of steam and hydrocarbon gas, preferably recycle hydrocarbon recovered from the fragmentation zone by retorting. The steam or mixture of steam and hydrocarbon of Ser. No. 641,815 is heated to a temperature in the range from 500 to about 1,000 F which have been found desirable for educing oil from fractured shale formations.

It is often the case that oil bearing reservoirs are sufficiently extensive to warrant the placement of two or more fragmentation zones in the same general area and in relatively close proximity to each other. The temperature of these detonation zones in the chimney is generally about 700l,000 F immediately following detonation. In view of the fact that it is desirable to heat steam and/or hydrocarbon in the subsequent stages of processing, e.g., during pretreatment of the chimney walls to reduce the permeability thereof, and during retorting, it is advantageous to make use of the heat stored in the detonation zones to heat the steam or hydrocarbon. For example, it is desirable to allow the temperature of a fragmentation zone to reduce to a level such as 300 F. prior to pretreatment. As a result, a substantial temperature differential exists between these relatively cool zones and recently fired chimneys. The advantage can be taken of this differential by passing low quality steam or water into the recently fired chimney to produce a higher quality steam which is removed via the top of the chimney and passed to the cooler" fragmentation zone which is to be pretreated. As a result, the temperature of the newly fired zones can be reduced rapidly while the heat removed therefrom can be utilized in pretreating or retorting fragmentation zones in a further state of development.

The application of this idea in one situation is illustrated by the following example.

EXAMPLE A 500 kiloton nuclear fission device is implanted in an oil shale strata having a vertical extent of 2,000 feet at a depth of 3,000 feet from the surface. The diameter of the nuclear chimney which will result from the detonation of this device is 572 feet with the major vertical axis extending 1,429 feet from the point of detonation to the upper extremity of the fragmentation zone. Chimney volume is 3.68 X 10 cubic feet which contains 1.85 X 10 tons of oil shale containing 1.10 X 10 barrels of reservoir oil. The initial chimney temperature following detonation is 200 F. This temperature is elevated to 400 F by the introduction of 400 F steam at a pressure of 250 p.s.i.g. and at a rate of 10 pounds per hour for a period of days after which the chimney sidewall permeability will be reduced 500 fold.

lclaim:

1. An improved method for recovering shale oil from oil shale formations by reducing the sidewall permeability of a subterranean detonation chimney containing fragmented shale within said chimney which comprises contacting said sidewall with steam at a temperature sufficiently above formation temperature but below the retorting temperature of said sidewall for a time sufficient to promote the desired degree of swelling of said sidewall whereby said permeability is reduced, and retorting the fragmented shale within said chimney by heating the fragmented shale to a temperature above the retorting temperature of said shale by contacting with a retorting gas heated to a temperature above the retorting temperature of the shale.

2. The method of claim 1 wherein said steam has a temperature within the range of from 350 to about 600 F. and a pressure offrom 100 to 1,500 p.s.i.a.

3. The method of claim 2 wherein the pressure of said steam is increased after the substantial reduction of said sidewall permeability to accelerate the selective reduction of residual permeability in said sidewall.

4. The method of claim 1 wherein said fragmented chimney shale is retorted by the passage of steam therethrough at a temperature of at least about 500 F. and at a quantity of at least about ,000 scf/ton of shale retorted.

5. The method of claim 1 wherein the shale comprising said sidewall has a thermal coefficient of expansion sufficient to effect at least a 50 fold reduction in permeability of said sidewall by increasing the temperature thereof to about 450 F.

6. The method of claim 1 wherein said shale is retorted by passing into the top of said fragmentation zone at least about 15,000 standard cubic feet of shale gas per ton of shale in said fragmentation zone at a temperature of least about 650 F. whereby hydrocarbon is educed from said shale.

7. The method of claim 8 wherein said hydrocarbon educed from said chimney is recovered by collecting educed hydrocarbon in the lower portion of said fragmentation zone and helping the thus accumulated hydrocarbon to the surface.

8. The method of claim 7 wherein at least a portion of said recovered educed hydrocarbon is heated to a temperature within the range of from about 650 to about l,0O0 F. and injected into said chimney to retort said shale.

' 9. The method of claim 1 wherein said steam is produced by passing water or steam into a first retorted subterranean fragmentation zone to produce steam of improved quality, said improved steam is recovered from said first zone and injected into a second fragmentation zone prior to retorting thereof, the temperature of said first zone being substantially in excess of the temperature of said second zone 'prior to passage of steam therethrough.

10. The method of claim 9 wherein the temperature of said first zone is within the range of from about 500 to about l,000 F. 

